Here is a diffuser panel in prototype form that I'm currently testing. I prefer to build prototypes first before coming up with a design that I will build properly.
The process can help with the design. In this case I want to make sure I like the result, and these light easy to make panels can be easily moved to try different placements.
As you can see, it is made with fins of ply and polystyrene wells stuck on with masking tape. They are all parts I had on hand. The panel is around 350 x 400mm and 55mm deep. It is effective to 2k with reduced effectiveness down to 1k.
This particular panel is designed for nearfield placement, around 300 - 500 mm. They will be placed on the rear wall against which my couch sits. In my initial tests I've compared to a bare wall and absorption. I prefer the sound of a bare wall over absorption, as the latter sounds artificially dead. I also prefer the panel over a bare wall as there is an added sense of ambience without adding anything unnatural. The effect with a small area is subtle.
I also have plans for a panel that is 110mm deep which will be effective to1k.
My plans are to add a very large panel to the ceiling and behind the listening couch, with smaller panels distributed around the room.